1. Field of the Invention
The present invention relates to heatpipes and heatpipe evaporators. More specifically, the present invention relates to increasing the surface area of an evaporator and simultaneously increasing the number of nucleation sites on the surface of an evaporator. Furthermore, the present invention uses a graphite composite to accomplish the above.
2. Summary of the Prior Art
Several prior art devices have been used to cool high power integrated circuits. They include simple approaches such as fans, and more complicated approaches involving elaborate heatpipes.
Referring to FIG. 1, a standard heatpipe and integrated circuit arrangement of the prior art is shown. The heatpipe 10 basically has two regions: a condenser 18 and an evaporator 17. The evaporator region 17 is usually comprised of a reservoir containing fluid 16 and an evaporating surface. This fluid 16 is boiled in the integrated circuit 12 cooling process and the vapors given off move through the heat tube of the heatpipe 10 to the condenser. The condenser region 18 is usually defined by the presence of heatpipe fins 19.
A heatpipe 10 may or may not have a wick. A wick is a mat or bundle of fibers or an arrangement of grooves or pores that is positioned from the evaporator region 17 to the condenser region 18. The wick collects condensation and returns it to the evaporator 17 through capillary action. The advantage of a heatpipe with a wick is that it may be positioned arbitrarily without regard for gravity. A wickless heatpipe or thermosyphion, on the other hand, must be placed upright so that condensed fluid flows back to the evaporator region 17.
As depicted from the cross-sectional view of FIG. 1, the evaporating surface is generally flat. This results in a minimum surface area and a minimum amount of fluid coming in contact with the evaporating surface. It is recognized in the prior art that by increasing the surface area, the amount of fluid contacting the evaporating surface is increased and, accordingly, the rate of cooling is increased.
In addition to increasing surface area, however, it is desirable to have a wealth of nucleation sites, microscopic re-entrant cavities that will retain small gas bubbles in the surface to initiate boiling at low wall superheats. Current techniques of increasing surface area including making cuts through the material, which is a metal, often copper, or molding the copper to have a varied surface. The greater the contours on the evaporator surface, however, the more difficult it is to operate on the surface to produce nucleation sites. For example, lasers have been used to roughen evaporator surfaces. But if grooves are cut into the evaporator to increase the surface area, it is exceedingly difficult to maneuver a laser into position to scar the walls of the cut grooves. Additionally, to the extent sufficient scarring is possible, it is a slow process and often has to be carried out one device at a time, as opposed to mass production.
Additionally, a new fiber has been developed which has interesting properties, including being over twice as thermally conductive as copper axially. These fibers are comprised of graphite.